Safari: RangeError: Maximum call stack size exceeded.

I'm not sure if it's valid to use br as a defaultParagraphSeparator, but when using that it generates an error on Safari on this line:

`if (firstChild && firstChild.nodeType === 3) exec(formatBlock, '<' + defaultParagraphSeparator + '>');else if (content.innerHTML === '
')`

It seems to work okay in other browsers.

Reproduce the Safari failure with `defaultParagraphSeparator` set to `br`, then inspect the reported `exec(formatBlock, '<' + defaultParagraphSeparator + '>')` branch and the adjacent `content.innerHTML === '
'` condition. Done means Safari no longer raises the maximum call stack error while paragraph editing continues to work.

Written by the indexing model from the issue text.